1. Field of the Invention
Embodiments of the present invention generally relate to digital communication systems that use multiple sub-carriers, and more particularly to systems and methods to detect and mitigate the effect of spurs in received sub-carriers in such systems, thereby improving system performance.
2. Description of the Related Art
Digital communication systems that use multiple sub-carriers are becoming increasingly prevalent in order to offer good performance under varying noise conditions. For example the IEEE 802.11 wireless standards employ a method known as Orthogonal Frequency Division Multiplexing (OFDM) to address multipath and other transmission impairments, and several ITU-T digital subscriber line (DSL) standards employ a similar method known as Discrete Multi-tone (DMT) to counter inter-symbol interference and other additive noises.
In an OFDM or DMT multiple sub-carrier system, a higher rate data signal may be divided among multiple narrowband sub-carriers that are orthogonal to one another in the frequency domain. The higher rate data signal may be transmitted as a set of parallel lower rate data signals each carried on a separate sub-carrier. In a wireless system, multipath may cause multiple versions of a transmitted data signal to arrive at a receiver with different delays, thereby resulting in inter-symbol interference created by received energy from different data signals transmitted at different times arriving at the receiver simultaneously. Each lower rate sub-carrier's symbol in an OFDM or DMT system may occupy a longer symbol period than in a higher rate single carrier system, and thus dispersion caused by multipath may be substantially contained within the longer symbol period, thereby reducing inter-symbol interference.
While a multiple sub-carrier system may transmit a set of symbols in parallel orthogonally, intervening transmission impairments may affect the orthogonality of the received sub-carrier symbols. To determine the effect of the transmission channel and impairments on receiver performance, the multiple sub-carrier system may use a set of training symbols to estimate the channel and noise. Subsequent data symbols, after the training symbols, may also be used to update the channel and noise estimates. The symbols received on each sub-carrier may be modified by the channel and noise estimates to improve detection and decoding performance.
To maintain time synchronization between the transmitter and the receiver in a multiple sub-carrier system, a number of sub-carriers, also known as “pilot” sub-carriers, may transmit a pre-determined pattern. Which specific sub-carriers are used for pilots may be fixed or may vary over time. For example, in an 802.11 system, four of the 52 orthogonal sub-carriers are dedicated as “pilot” subcarriers; while in an ISDB-T digital TV system, a number of sub-carriers are used to transmit “pilot” symbols at regular intervals and transmit data symbols at other times.
Narrowband noise impairments, also called spurs, on the “pilot” sub-carriers may affect the time synchronization recovery in the receiver and thereby may affect system performance, while spurs on the “data” sub-carriers may affect decoding of the data by the receiver. In some systems, the presence and location of a narrowband interferer may be known a priori, as described in U.S. Pat. No. 7,321,631 assigned to Atheros Communications and incorporated by reference herein. For example, a system's reference oscillator may create harmonics at odd and even multiples of the reference frequency that may couple into and adversely affect the performance of a communication system's receiver. By examining how a noise spur may affect information transmitted on a set of sub-carriers, a metric may be associated with each sub-carrier prior to using symbols received in those sub-carriers for time synchronization or data decoding. One such metric known as “channel state information” (CSI) may determine a weighting given to bits of a received symbol on a sub-carrier based on the transmitted data rate for that subcarrier, and/or on the estimated channel response, and/or on the measured noise on that sub-carrier. The weightings given to bits on sub-carriers adjacent to a sub-carrier containing significant channel attenuation or additive noise may also be adjusted. A Viterbi decoder may then use the CSI metric to “weight” its decoding decisions by de-emphasizing data received on sub-carriers with significant attenuation or measured noise. Similarly a timing synchronization routine may de-emphasize or ignore the information on pilot sub-carriers containing significant attenuation or measured noise.
In many systems the location of narrowband interference may not be known in advance or may vary during transmission, so a method to detect adaptively the presence and location of such spurs and mitigate their effects to improve system performance in communication systems using multiple sub-carriers is needed.